The Large Movie Review Dataset (aclImdb v1.0): 50,000 polar movie reviews from IMDB, split evenly for training and testing. For over a decade the default benchmark for binary sentiment classification — simple enough to load with one function call, large enough that word order and negation actually matter.
Provenance
Collected at Stanford and released alongside Maas, Daly, Pham, Huang, Ng and Potts, Learning Word Vectors for Sentiment Analysis, ACL 2011. The dataset page (and download) lives at ai.stanford.edu/~amaas/data/sentiment. The original motivation was learning sentiment-aware word representations; the review corpus it shipped with promptly outlived the method.